Skip transform type search.

Without txk_sel:
Skip remaining transform type search when all transform blocks inside
the coding block have eob = 0.

With txk_sel:
For each transform block, whenever eob = 0, we skip remaining
transform type search.

Speed impact:
On low bitrate, 25% speed up.
On high bitrate, 15-20% speed up.

Performance impact: Google test lowres, 30 frames
With txk_sel: 0.15% drop
Without txk_sel: 0.30% drop

Change-Id: I5e8db730a19feec22e378611046b1ce1ab001c85